A typical liquid cryogen delivery system for cryogenic freezing tunnels comprises elevated pressurized storage tanks, thermally insulated transfer lines, control valves, liquid-vapor phase separators and spray header assemblies. Because of the warming which occurs in the input system and pressure drops due to flow through the transfer lines and control valves, one of the problems of such tunnels is that a substantial volume of the cryogen, for example, liquid nitrogen (LIN), becomes converted to vapor by the time the cryogen enters the spray header assembly. The resulting two-phase mixture of vapor and LIN does not move in a smooth and predictable fashion, but rather tends to flow in volumetrically small slugs of LIN interspersed with volumetrically large slugs of vapor. When the stream of LIN having this heterogeneous unsteady flow is introduced into the spray header manifold, the product to be frozen is contacted alternately by liquid and then gaseous cryogen. Since liquid nitrogen has about twice the available refrigeration of gaseous nitrogen, this condition results in the most significant operating problem in freezing of the product, i.e. variations in the temperature of the frozen product.
Alternate delivery of liquid and gaseous nitrogen through the spray header causes an additional operating problem. The liquid and gaseous nitrogen pass through the orifices of the spray header at a different mass flow rate. The gas flow control system moves a volume of gas through the freezing tunnel at a rate equal to the liquid nitrogen flow rate. Gaseous nitrogen flows through the spray header at a much lower flow rate, causing room air to enter the freezer discharge opening. Room air entering the freezer will result in frost accumulation within the freezer and will also add an excessive heat input to the freezer.
Another problem of such freezers is that the conventional LIN transfer line may not deliver the same quality LIN to each freezer, especially if the horizontal portion of the transfer line is not perfectly level. The freezers receiving the poorest quality LIN, i.e. primarily gaseous nitrogen, will experience severe problems in freezing products such as food at the design production rate.
A conventional liquid-vapor phase separator in a cryogen delivery system consists of an insulated vessel, a bottom liquid withdrawal line, a top gas venting line, and a liquid level sensor. The level sensor is used to actuate a solenoid valve in the gas venting line. The pressure inside the insulated vessel will vary because of the opening and closing of the solenoid valve. Thus, the spray header mounted on the liquid withdrawal line will experience fluctuating pressure and varying liquid flow rates, which will result in non-uniform freezing of the products, which is especially a problem in food freezing.
Spray headers are found in the cryogenic freezing tunnels of the type described in the following U.S. Pat Nos., Berreth et al., 3,403,527; Flynn et al., 3,583,171; Klee et al., 3,613,386; Klee, RE. 28,712; Klee et al., 3,813,895; Klee et al., 3,892,104; and Morgan 4,171,625. The Morgan et al. reference is the only one which addresses the problem of venting the build-up of cryogenic vapors in the cryogen delivery system and a solution thereto. In column 9, line 36 through column 10, line 3 of this reference, it is disclosed that spray header 156 serves as a phase separator for the gas and liquid phases of the cryogen and gas relief nozzles 160 having a given internal diameter. The nozzles are mounted on the top of the cryogen delivery tubes to allow the gas to escape. This creates an even flow of liquid out of the bottom and side of header 156 onto the products within the freezing tunnel below. The fixed diameter of the relief nozzles has the disadvantage of providing for the escape of only a fixed amount per unit time of vaporized cryogen. The diameter would have to be changed for changes in the cryogen flow rate or in the percent of cryogen that becomes vaporized due to pressure variations between the cryogen storage tank and the spray header or in the amount or type of insulation used throughout the cryogen delivery system in order to maintain the desired even flow of liquid onto the products within the freezing tunnel.
One method of overcoming the disadvantages of the relief nozzles of Morgan et al. is disclosed in NASA TECH BRIEF No. 66-10136 dated April 1966. The cryogenic trap valve disclosed in this reference comprises an aluminum valve housing and an Invar alloy valve stem. When this temperature actuated valve is filled with cryogenic liquid, the aluminum body is in its maximum contraction and the valve stem is seated in the outlet orifice of the valve. As the liquid vaporizes and the valve housing increases in temperature, the housing will expand while the Invar alloy stem remains of substantially the same dimension. This will allow the vapors to escape through the outlet orifice. This valve has substantially only one mode of venting capabilities, from a fully closed to a fully opened position. The slight movement of the valve body of this prior art valve, as the vapors increase in temperature, produces only a momentary throttling effect. The net effect of such a movement is to cause the device to operate essentially as an opened/closed system.